1. Technical Field
The present invention relates generally to fastened joints and more particularly to a method for improving the sealing between the members of a fastened joint by directing the transmission of clamping forces into critical sealing areas.
2. Discussion
Vehicle manufacturers have undertaken substantial efforts over the last decade in an attempt to improve the sealing between members of a fastened joint and eliminate the possibility of a leak between the joint members. Once a persistent leak path has been identified, there is frequently little or no correlation between the amount of resources spent on improving the sealing characteristics of the joint and the amount of expenses incurred through warranty. This is primarily due to the impact of a leak on the consumer's perception of the vehicle. Once a leak has been identified, the consumer will typically attribute a lack of quality to the vehicle generally, rendering it more likely that the consumer will complain to the vehicle dealer and to other consumers. Accordingly, vehicle manufacturers have put a priority on eliminating leaks in an effort to improve the image of their products.
These efforts have primarily focused around the magnitude of residual clamping force which is directed though the sealing area of the joint. One approach has been to maintain the magnitude of residual clamping force above a predetermined minimum level over the entire sealing area. The methods employed under this approach have included improved fastening strategies, increasing the number and/or size of the fasteners, utilizing higher grade fasteners, changing the geometry of the fastener or utilizing specialized gaskets which concentrate clamping force along a desired path. Another approach has been to simply acknowledge that the creation of a leak path is due to an inadequate clamping force exerted over the sealing area and to employ specialized gaskets or sealing compounds (e.g., room temperature vulcanizing silicone or anaerobic sealants) that are more tolerant of such variations in the clamping force. While these methods are often successful, they are attendant with several drawbacks.
One such drawback concerns the impact on those who will either assemble or service the fastened joint. The above mentioned methods, particularly those that employ improved fastening strategies, fasteners of several different sizes or grades or sealing compounds, complicate the assembly or servicing of the joint to some degree. Additional complexity in the assembly or servicing of the joint, especially where the operation is performed on a high volume assembly line, increases the risk that the technician performing the operation will make a mistake, with the results tending to be more catastrophic in nature than a leak. Complicating matters is the fact that often times there is no means available by which one can determine if the assembly/servicing method was performed correctly. For example, a person inspecting the work of the technician after the joint has been fastened cannot reliably determine whether the fasteners have been tightened in the proper sequence or if the proper amount of sealing compound has been utilized.
The primary drawback, however, is the cost associated with each of the above-mentioned methods. These costs are readily discernable where a change is made to the number, size, quantity or geometry of the fasteners; the differential cost is easily calculated from the increased cost of the components added to the additional labor costs, if any. The use of specialized gaskets and sealing compounds can be analyzed in a similar manner. However, restricting the analysis to recurring direct costs prevents the impact of many factors from being considered.
For example, implementation of an improved fastening strategy can require the use of new, more specialized and frequently less efficient fastening tools, especially where torque-turn or yield type fastening strategies are utilized. In addition to the new fastening equipment, new equipment for spot-checking the operation of the fastening process, such as ultrasonic measuring equipment or precision micrometers, may be required. As such, consideration should also be given to cost of purchasing the production and auditing equipment, the recurring maintenance and training costs associated with this equipment and the impact on the efficiency of both direct (i.e., production) and indirect (i.e., auditing) labor.
Consequently, there remains a need in the art for a method for improving the sealing between members of a fastened joint which does not result in a substantial cost and which can be easily implemented without specialized assembly equipment, additional components or materials, or complicated assembly steps.